package com.gqb.wangyi;

import java.util.Scanner;

public class Main2 {
	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		while(scanner.hasNext()){
			int courseCount=0,manfen=0,avg=0;
			courseCount=scanner.nextInt();
			manfen=scanner.nextInt();
			avg=scanner.nextInt();
			Integer[][] a=new Integer[courseCount][courseCount];
			for (int i = 0; i < courseCount; i++) {
				a[i][0]=scanner.nextInt();
				a[i][1]=scanner.nextInt();
			}
			int fenshu=courseCount*avg;
			long learntTime=0;
			for (int i = 0; i < a.length; i++) {
				fenshu-=a[i][0];
			}
			a=sort(a);
			for (int i = 0; i < a.length; i++) {
				if(fenshu>0){
					if(manfen-a[i][0]>0){
						if(fenshu-(manfen-a[i][0])>0){
							learntTime+=(manfen-a[i][0])*a[i][1];
							fenshu-=manfen-a[i][0];
						}else{
							learntTime+=fenshu*a[i][1];
							fenshu=0;
						}
					}
				}
			}
			System.out.println(learntTime);
		}
		scanner.close();
	}
	
	public static Integer[][] sort(Integer[][] a){
		for (int i = 0; i < a.length; i++) {
			for (int j = i + 1; j < a.length; j++) {
				int temp = 0;
				if (a[i][1] > a[j][1]) {
					temp = a[i][1];
					a[i][1]=a[j][1];
					a[j][1]=temp;
					temp = a[i][0];
					a[i][0]=a[j][0];
					a[j][0]=temp;
				}
			}
		}
		return  a;
	}
}
